Rickettsia prowazeki, the etiological agent of epidemic typhus, is unusual, even amongst the intracellular bacterial parasites, inthat it grows in the cytoplasm, rather than in a vacuole, of its eukaryotic host. Hence, our focus has been to ask what are the biochemical and physiological mechanisms by which this organism copes whith the problems and exploits the opportunities of this unique environmental niche and what are the mechanisms by which the rickettsiae enter the host cell's cytoplasm. There are three general areas that we intend to investigate during the next five years. 1) The transport systems for those nutrients that the rickettsia encounters in the cytoplasm. This area includes the study of NAD(H), ATP/ADP and amino acids translocases and the development of a membrane vesicle system. 2) The metabolic capabilities of R. prowazeki and the regulation of these processes. The role of ATP/ADP in regulation of rickettsial enzymes and use of somatic cell mutants to explore rickettsial metabolism are key features of this area. 3) The penetration of eukaryotic cells by the rickettsia as determined both in a model system with erythrocytes and in a productive system with competent host cells. The role of the phospholipase A activity and an elucidation of binding sites in rickettsia-host interactions are included in this area. All these areas are highly interrelated and projects with a lengthy development phase are interspersed with those that are straight forward.